(1) Field of the Invention
The present invention relates to a roller-shaped heater which is composed of resistance heating elements in a sheet-like form and is arranged in close contact with a roller (e.g., the inner peripheral surface of the roller) and a fusing unit using the roller-shaped heater.
(2) Description of the Prior Art
In image forming apparatus using electrophotography, a photoreceptor is charged uniformly, then is illuminated by an exposure light source such as a laser beam etc., in accordance with image information so as to form a static latent image on the photoreceptor. Next, the static latent image on the photoreceptor is developed with a developer into a visual image. The developed image is then transferred to a recording medium such as recording paper etc., through a transfer station. Then the image on the recording paper is fused and fused by a fusing unit.
The fusing unit includes, for example, a pair of rollers opposing to each other, i.e., a heat roller and a pressure roller. The toner image on the recording paper is fed and pressed with heat between the two rollers onto the recording paper. The heat roller is composed of, for example, a cylindrical metallic core made up of aluminum etc., and a separation layer of a fluororesin coated on the outer peripheral surface of the metal core. The metal core generally incorporates a halogen lamp as a heater. When electric power is supplied to this halogen lamp, it generates heat and the heat makes the entire heat roller hot.
However, because there is an air layer between the halogen lamp and the metal core, the conventional heat roller using a halogen lamp has suffered a drawback in that the efficiency of heat transport to the heat roller is poor and hence the heat roller takes time until it is heated to a required temperature level.
In order to solve this problem, there is a proposed method in which a resistance heater which itself heats is arranged on the inner peripheral surface of a heat roller so that the heat roller will be heated directly with no air layer in between. A heat roller of this type is disclosed in Japanese Patent Application Laid-Open Hei 8 No.220915. In Japanese Patent Application Laid-Open Hei 8 No.220915, a resistance heating element having a predetermined pattern is formed on an insulation sheet. This is referred to as a heat generator (heat generating sheet). The heat generator (heat generating sheet) is inserted into the heat roller interior with the resistance heating element oriented toward the central axis of the heat roller. The resistance heating element has a pair of electric power supply strips disposed at both ends of the roller body and extending in the peripheral direction of the roller body and the power supply portions electrically connected to the resistance heating element is bonded, thus constituting the heat generator (heat generating sheet). A pair of electrodes are placed in frictional contact with these electric power supply portions so as to supply electric power to the resistance heating element via the power supply portions. Thus, the resistance heating element is supplied with power to heat itself and hence heat the entire roller.
In the case of using a resistance heating element of the above type, setting of a heat generator (heat generating sheet) inside the heat roller becomes difficult as the roller diameter becomes smaller. Further, in this method, when the resistance heating element on the heat generator (heat generating sheet) is supplied with electric power, the entire sheet is heated. Since heating only in the nip area is effective enough to fuse the recording medium such as recording paper etc., heating the entire heat roller is wasteful and consumes more electric power compared to a which the roller is heated only around the nip area.